I have several unpaid medical bills that are in collections. Upon the collection agency contacting me I worked out a payment plan over the phone. After making a few payments I called the case worker and asked for her to provide the current balance of my account. I was then told to include a letter along with my payment asking for balance details in writing. I mailed the check with a letter requesting my balance details and receipt of payment. I have not received any paperwork regarding my balance or receipt of payment after several attempts to do so. How should I handle this?